This invention relates to an automatic sewing machine controlled by a computer for producing a path curve or trajectory as a relative movement between a sewing head with a needle and workpieces to be sewn.
An automatic sewing machine is known from U.S. Pat. No. 4,312,283 in which the sewing head can be moved in the X-direction and the Y-direction by means of computer-controlled servomotors. A workpiece to be sewn is held in a workpiece holder, the computer being programmed in such a way that sewing takes place along the trajectory in accordance with a previously fed-in program. When the sewing process is at an end, the sewn workpiece is removed from the workpiece holder, after which a new workpiece is inserted and the sewing process is repeated.
The technical information sheet entitled "Electronic Controlled Stitcher" of the International Shoe Machine Corporation, Nashua, New Hampshire, U.S.A., discloses an electronically controlled automatic sewing machine, in which the sewing head is fixed. Two displaceable workpiece clamps are provided, which are moved in alternating manner beneath the sewing head, where a seam with a predetermined course is produced in accordance with a previously fed-in program. During this time, the workpiece can be removed from the other workpiece clamp and a new blank can be inserted. When the two workpiece clamps are firmly joined together, i.e. when the workpiece clamp to be charged is also moved with the workpiece clamp located below the sewing head, this construction is largely unusable, because it is virtually impossible accurately to introduce a blank. If, however, the workpiece clamp to be charged is disengaged from the workpiece clamp beneath the sewing head, then considerable constructional effort and expenditure are required for this.
In addition, the often very abrupt reciprocating movements of the workpiece clamps with the workpiece, so-called "shaking", are disadvantageous for the precision of the sewing process and particularly if a seam is to be made in the inner area of the workpiece surface. The reason is that the workpieces are made from flexible material and in the case of such shaking movements, displacements of the workpiece can easily take place.
U.S. Pat. No. 3,878,801 discloses a program-controlled automatic sewing machine, whose sewing head is fixed. A charging station is provided, into which blanks are inserted in spaced manner from the sewing head and these are subsequently transferred into a working station, which guides the workpiece under the sewing head, in accordance with the predetermined trajectory, accompanied by the production of a seam. Even at the time of transferring the workpieces, displacements occur. In addition, only a central clamping of the particular workpiece is possible so that further displacements are possible.